A "Gunn diode" is a semiconductor device extensively employed in microwave and millimeter wave sources because of its property of oscillating at high frequencies. Gunn diodes are manufactured from semiconductor materials that are known to exhibit the transferred electron effect, which gives rise to a negative differential resistivity occurring in bulk semiconductor material (i.e. not at a junction) when a voltage, which is at least equal to the threshold voltage for a given material, is applied to a properly prepared sample of such material. The transferred electron effect, also known as the Gunn effect, can be used to produce a frequency of oscillation inversely proportional to the time it takes for an electron to cross a sample of such semiconductor material.
The Gunn effect is found in so-called III-V compound semiconductors which comprise combinations of Ga, In, and Al from group III of the periodic table of elements, and P, As, and Sb from group V. Of these materials, Gallium Arsenide (GaAs) has been the most highly developed for commercial applications because this material is relatively easy to work with. However, Indium Phosphide (InP) is known to have superior electronic properties that are especially advantageous for high-frequency millimeter-wave applications. Although, in theory, InP Gunn diode oscillators can be made to produce higher output power at higher frequencies than GaAs devices, InP has not been widely employed in commercial-scale devices due to its poor mechanical properties such as low thermal metallurgical interface stability, poor mechanical strength, and problems associated with its processing.
More specifically, a typical Gunn diode fabrication process includes growing a crystalline substrate of the desired semiconductor material and then growing additional layers, doped appropriately, onto the substrate. The substrate is used as a foundation upon which many thousands of individual devices are grown. The devices are ultimately cut apart, and, in this process, most of the substrate material is destroyed. This process is not readily adaptable to fabrication of InP devices because, in part, InP substrates show a tendency to decompose at high temperatures which are employed during material growth. In addition, InP is more expensive than GaAs, and since the substrate material is largely discarded, devices that are grown on InP substrates cannot compete cost-effectively with GaAs devices on a per-area basis. Another problem associated with fabrication of InP devices is that device contacts are difficult to attach because the preferred contact attachment techniques involve ultrasonic bonding processes that may cause the fragile InP substrate to fracture and be destroyed.
Presently, the design of commercial InP Gunn diodes is essentially analogous to traditional GaAs Gunn diode design, which dates back about 20 years. A critical step in the fabrication of such devices involves precision thinning of the InP substrate to a thickness of around 10 .mu.m in order to create a more uniform current density and electric field profile in the active layer. For efficient very high frequency operation, such as 140 GHz, consideration of the skin effect necessitates a uniform substrate thickness of about 5 .mu.m. The substrate thinning operation is a complicated and expensive process. Furthermore, commercial InP Gunn diodes, as well as experimental devices reported in the literature, typically employ a plated heat sink geometry. However, extensive device modeling has shown that a flip-chip style device, also referred to as an Ultrasonic Thermo-Compression (UTC) bonded device in the literature, wherein the thin-metalized cathode is UTC bonded face-down directly to the package stud, without a large integral plated heat sink, provides a lower overall thermal resistance. Such flip-chip structures cannot be used with the conventional InP diode design because, as indicated, a very delicate semiconductor layer left after the precision thinning process cannot withstand the mechanical stresses induced during the ultrasonic die bonding process used for manufacturing flip-chip devices.
The present state of the art in design and manufacturing of the InP gunn diodes is disclosed in J. Crowley et al., "Manufacturing Method and Technology Project for Millimeter-Wave InP Gunn Devices at 56 and 94 GHz," Research and Development Technical Report DELET-TR-82-C-0386, which discloses the research efforts of the leading manufacturer of InP devices. On page 18, the report indicates that the integrated heat sink (IHS) style device is the only known practical InP Gunn device configuration, although other configurations may provide a lower thermal resistance in the device. Also, on the same page, the report indicates that in a traditional device, the fragile nature of InP mesas precludes ultrasonic-thermocompression bonding during a high-yield process. Further, on page 23, the report indicates that a key factor in fabricating the highest efficiency device is the reduction of parasitic resistance, which is, at least partially, due to DC and skin effect resistance of the substrate material of the conventional device. According to the research results set forth in the report, the best approach to minimizing the series resistance is eliminating as much of the substrate material as possible.
The above discussion demonstrates that it is desirable to avoid the use of an expensive and fragile InP substrate material during device manufacture. Also, it is desirable to produce a diode structure in which the electrical resistance and power dissipation due to the substrate layer is eliminated or substantially reduced.
InP epitaxial layers can be grown on a GaAs substrate using vapor phase epitaxy and the electronic characteristics of InP layers can be controlled by doping with sulphur or other elements. See, Teng, "InP Devices on GaAs Substrates" Microwave Journal, December 1985, pp. 138-140, incorporated herein by reference. The Teng article does not teach the steps necessary to form a useful device. According to Teng, a device would necessarily include a GaAs substrate with InP layers formed thereon. This would preclude use of a layer of metallization to create contacts on both sides of the InP layers and, therefore, the GaAs substrate would be part of the thermal and/or electrical path from the active InP region to one of the device contacts and would contribute to the parasitic electrical and thermal resistance in the device. Since GaAs is not a good conductor of heat, such devices may have limited thermal dissipation capability and thus limited power handling capacity.
Thus, while the art indicates that formation of InP epitaxial layers on GaAs substrates is possible, the art does not teach a useful method of manufacturing InP devices using GaAs substrates. The art does not teach a method that allows for manufacture of Gunn diodes which do not contain an effective substrate layer and thereby provide a more direct electrical connection to the active layer. Furthermore, an improved electrical connection to the active layer is also desirable in conventional GaAs Gunn diodes, which contain a GaAs substrate layer.